block: remove 'x' prefix from experimental bitmap APIs

The 'x' prefix was added because I was uncertain of the direction we'd
take for the libvirt API. With the general approach solidified, I feel
comfortable committing to this API for 4.0.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20181221093529.23855-5-jsnow@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>